In sheet-feed offset printing machines, the ink to be printed is fed to a printing plate from an ink fountain roller via a vibrator roller, via ink rollers arranged downstream of the vibrator roller and ink applicator rollers. A number of ink metering elements which can be set individually and which correspond to ink-metering zones interact with the ink fountain roller. Such metering elements may be remotely controllable via associated drives. In this manner, the ink requirement in the individual ink metering zones corresponding to regions of the printing plate can be set in order to achieve the desired inking. Furthermore, the vibrator cycle and the vibrator strip width of the vibrator roller may be selected to further influence the ink feed. Inking units of sheet-fed offset printing machines usually also exhibit transverse ink flow. In other words, a quantity of ink fed in one ink-metering zone will generally influence to some extent the inking in adjacent metering zones. This transverse ink flow is further reinforced, depending on the subject to be printed, by the lateral distribution, which can also be set.
In the simplest case, the ink feed is set by hand according to an estimate by the printer, using the ink metering elements and vibrator roller. Thus, the ink metering elements and the settings on the vibrator roller, i.e cycle, and strip width, are set appropriately following a visual inspection of the subject printing plate. Automation of these presetting measures may be obtained using printing-plate readers or by using printing plate data derived from the pre-press stage. In a printing-plate reader, the proportion of the printing area in the individual ink metering zones is ascertained by recording the reflectance of the plate. Then, the settings of the ink metering elements are derived from the proportions of the printing area in the metering zones in accordance with predefined characteristic curves. Settings for the vibrator cycle and the vibrator strip width can be derived from the average ink requirement over all the ink metering zones, also known as the format width.
In the case of ascertaining ink pre-setting data from the pre-press stage, either the image data are used directly, or coarse-screen data are used to derive the proportion of the printing area. Here, too, the determination of the proportion of the printing area is carried out with respect to ink metering zones. Similarly, the determination of the vibrator cycle and vibrator strip width is carried out by taking into account the proportion of the printing area over all of the ink metering zones of the subject.
Because of the large number of rollers, the inking process of sheet-fed offset printing machines exhibits a pronounced inertia. That is, a change to the ink feed requires a certain amount of time and a certain number of imperfectly printed sheets before the desired inking change appears on the printed material. When adjustments are being made for a new print job, frequent adjustments in the ink feed are required until the intended inking is achieved. Since each change to the inking causes rejects, it is desirable that the number of inking changes, and thus the number of reject sheets, is kept as low as possible. The number of adjustments can be minimized by means of good ink pre-setting using pre-setting data. However, subsequent necessary inking changes will still produce reject sheets until a stable inking state is achieved. The time duration of this transient is increased if the proportion of the printing area is low; that is to say an inking change is established particularly slowly in the metering zones having a low proportion of the overall printing area.
DE 44 29 481 C2 discloses a method for feeding printing ink into a roller inking unit of a printing machine, in which, at least in one of the metering zones having a proportion of the printing area which is different from zero, the corresponding metering element is at least partly closed. An aspect of this method provides that, in the case of a low proportion of the printing area overall on the printing plate, only each second ink metering zone is opened, and the ink metering elements located in between always remain closed. This results in a higher ink flow in the opened ink metering zones, with the corresponding excess of ink being conveyed into the ink metering zones corresponding to closed ink meters.
However, this previously disclosed method can be used only when producing printed products in which the printing plate overall, that is to say over the entire format width, has a low proportion of printed area as compared to non-printed area. However, in many print jobs, subject regions containing a high proportion of printing area lie alongside subject regions having a low proportion of printing area. Accordingly, there exists a need for a method of metering inking in such circumstances to improve the transient response, and to minimize the number of reject sheets produced after inking setting changes.